Zeolites and zeolite-like materials do not comprise an easily definable family of crystalline solids. However, the Structure Commission of the International Zeolite Association has presently approved more than 200 different zeolite framework types and assigned a 3-letter code to each framework. A criterion for distinguishing zeolites and zeolite-like materials from denser tectosilicates is based on the framework density, the number of tetrahedrally coordinated framework atoms per 1000 Å3. The tetrahedrally coordinated framework atoms are also denoted T-atoms. The maximum framework density for zeolites and zeolite-like materials ranges from 19 to over 21 tetrahedrally coordinated framework atoms per 1000 Å3, depending on the type of smallest ring present, whereas the minimum for denser structures ranges from 20 to 22. The Structure Commission maintains a zeolite structure database accessible via the internet [http://www.iza-structure.org/] and is also regularly revising and publishing the Atlas of Zeolite Framework Types. The 6th revised edition of the Atlas was published in 2007 [Ch. Baerlocher, L. B. Mc Cusker, D. H. Olson. Atlas of Zeolite Framework Types, 6th Ed., 2007, Elsevier, ISBN 978-0-444-53064-6].
Zeolite frameworks are built from TO4 tetrahedra and the T-atoms are usually silicon and aluminium atoms, but zeolite frameworks can also be prepared from only SiO4 thetrahedra. In the aluminophosphates (AlPO4), the T atoms are aluminium and phosphorous atoms. However, there are many more possibilities and atoms such as Si, Al, P, Ga, Ge, B, Be, Ti, Fe etc. can serve as T-atoms in zeolite frameworks. Zeolites and zeolite-like materials are microporous solids also known as “molecular sieves.” The term molecular sieve refers to a particular property of these materials, i.e., the ability to selectively separate molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The dimensions of the channels control the maximum size of the molecular or ionic species that can enter the pores of a zeolite. The aperture of the channels are conventionally defined by the ring size, where, for example, the term “8-ring” refers to a closed loop that is built from 8 T-atoms and 8 oxygen atoms.
Zeolites and zeolite like materials have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions can readily be exchanged, which explains why zeolites can serve as ion exchangers.
Natural zeolite minerals are usually formed where volcanic rocks and ash layers react with alkaline groundwater. Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz, or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential. Some of the more common zeolites found as minerals in nature are (3-letter codes within brackets) analcime (ANA), chabazite (CHA), clinoptilolite (HEU), heulandite (HEU), natrolite (NAT), phillipsite (PHI), and stilbite (STI).
Zeolites and zeolite like materials can also be prepared synthetically. A frequently prepared zeolite framework is the MFI framework, which has 10 T-atoms in the ring and thereby a suitable pore size for many applications. This framework can be prepared in pure silica form, i.e. the T-atoms are only silicon atoms. In this case, the structure is denoted silicalite-1. However, if some of the silicon atoms are replaced with aluminium atoms, the structure is denoted ZSM-5. Templates or structure directing agents are added to the reaction mixture in the synthesis of zeolites and zeolite like materials to direct the crystallization to the desired framework. For example tetrapropyl ammonium hydroxide is often used as a template in the synthesis of MFI zeolite.
Hydroxide ions are usually employed as a mineralizing agent in the hydrothermal synthesis of these materials. The fluoride route to the synthesis of these materials is based on the substitution of fluoride anions for hydroxide anions implying that fluoride takes charge of the mineralizing role that OH− has in the conventional synthesis route (the OH− route).
The source of starting materials employed for preparation of zeolites and zeolite-like materials may be selected from all compounds that are sufficiently reactive to produce the framework structure chemically. Suitable silica sources are for example silicon alkoxides, hydrated silicates, precipitated silica powders, fumed silica and colloidal silica sols. In the hydroxide ion route, the necessary alkalinity is supplied by addition of alkali hydroxides, alkaline earth hydroxides or organic bases or combinations thereof to the aqueous reaction mixture.
The structure directing agents commonly used may also assist in controlling pH of the mixture or pH is adjusted usually by addition of alkali hydroxides. The synthesis pH in hydroxide media is typically about 11 or above whilst it is typically lower in fluoride media (6-11). In the fluoride route of the synthesis, the fluoride anion is suitably supplied to the reaction mixture by any fluorine-containing compound, which ionizes to a sufficient degree in the reaction mixture.
The synthesis of zeolites and zeolite-like materials using fluoride has usually been described with emphasis on the effect of the presence of fluoride on the crystal size, the low amount of defects in the zeolite, the substitution of silicon by trivalent or tetravalent elements and the importance of the organic species leading to a given material. In general, the fluoride route of synthesis has been shown to result in large crystals (typically larger than several micrometers) with a very low degree of connectivity defects.
The first clear example of the use of fluoride in the synthesis of silicalite-1 in slightly alkaline media was disclosed by Flanigen and Patton: U.S. Pat. No. 4,073,865 (1978) describes the preparation of a novel crystalline silica polymorph by a hydrothermal process in which fluoride anions were included in the reaction mixture. The silica polymorph was e.g. prepared from a reaction mixture having a pH of preferably 7.4-10, containing 150 to 1500 moles of H2O, from 13 to 50 moles of SiO2, from 2 to 12 moles of fluoride ions and from 0 to 6 moles of an alkali metal oxide. Each of these reagents were present per 2 moles of a quaternary ammonium cation. The length of the rod-shaped crystals could be as large as 200 micrometers and exhibited higher hydrophobicity than silicalite-1 zeolite synthesized as described in U.S. Pat. No. 4,061,724 (1977), using OH— as mineralizer.
T. Kida et al. (Ceramics International 30 (2004) 727-732) synthesized large silicalite-1 crystals by hydrothermal treatment of a reaction mixture (SiO2-TPABr—NH4F—H2O) at 150-200° C. Large crystals up to 1800 micrometer were obtained.
Benoit Louis and Lioubov Kiwi-Minsker (Micropor. Mesopor. Mater. 74 (2004) 171-178) prepared ZSM-5 zeolites at neutral pH by fluoride-mediated synthesis. By varying the F/Si ratio from 0.3 to 1.6, the size of the zeolite crystals could be tailored between 10 and 75 micrometers.
R. Mostowicz et al. (Zeolites 13 (1993) 678-684) used a variable HF/NaF ratio to synthesize zeolite of MFI structure and obtained regularly sized crystals with a length between 80 and 120 micrometers.
J. Patarin et al. (Zeolites 10 (1990) 674-679) synthesized MFI-type ferrisilicate zeolite (the mole ratio of Si/Fe varied from 28 to 10,000) in the presence of fluoride ions by using different silicon and iron sources. Iron was homogeneously distributed in the crystals and the crystal size varied from 50 to 80 micrometers.
M. Veltri et al. (Micropor. Mesopor. Mater. 420 (2004) 145-154) synthesized V-MFI zeolite crystals by hydrothermal treatment of a reaction mixture (Na2O—VO2—NaF—SO3—SiO2-TPABr—H2O) at 190° C. The size of the synthesized crystals varied from 11.5 to 33.5 micrometers.
In the art, the replacement of the hydroxide anions by fluoride anions as mineralizing agent is known to facilitate the synthesis of large zeolite crystals.
Methods for the preparation of small zeolite crystals have been disclosed in the patent literature. The first example was disclosed by Sterte: U.S. Pat. No. 5,863,516 (A) describes the preparation of colloidal zeolite suspensions using hydroxide as the mineralizing agent.
There are numerous publications in the scientific literature describing the preparation of colloidal zeolite crystals using hydroxide as mineralizing agent. A review has been published by Tosheva and Valtchev [Nanozeolites: Synthesis, Crystallization Mechanism, and Applications, Lubomira Tosheva and Valentin P. Valtchev*. Chem. Mater. 2005, 17, 2494-2513]. It is known in the art that high supersaturation and steric stabilization of the nuclei in the reaction mixture are key factors for the formation of nonaggregated zeolite nanocrystals. These conditions are usually achieved when the concentration of the organic structure directing agent is high and the concentration of alkali cations is very low in the reaction mixture. Under these conditions, the negatively charged subcolloidal particles are not aggregated in the reaction mixture. Furthermore, hydrothermal treatment at relatively low temperature is necessary to obtain small zeolite crystals [Nanozeolites: Synthesis, Crystallization Mechanism, and Applications, Lubomira Tosheva and Valentin P. Valtchev*. Chem. Mater. 2005, 17, 2494-2513].
There is still a need in the art to identify methods for producing small zeolite and/or zeolite-like crystals with a low amount of defects. Such crystals are particularly useful for producing thin films and membranes as well as in the manufacture of catalysts and adsorbents. Small zeolite and/or zeolite-like crystals with a low amount of defects may also be used directly as catalysts and adsorbents. Further, it is also a need in the art to identify a method, which produces membranes that have fewer defects, are hydrophobic and thereby better adapted for separating non-polar components from polar components.